Method and device for reducing the separation of volatile organic compounds from oil filling of tanks

ABSTRACT

A method and a device for reducing the evaporation of volatile organic compounds (VOC) when an essentially liquid petroleum product is being filled into a storage and/or transport tank ( 16 ) through a drop line ( 14 ), possibly also through a loading point ( 8 ), in which a gas or mixture of gases is supplied to the upper portion of the drop line ( 14 ) or the loading point ( 8 ) during the filling operation.

[0001] This invention relates to a method for reducing the fraction of volatile organic compounds (VOC) that separate from oil, especially from crude oil, during the filling of larger tanks, as is normal for example when a ship is being loaded from a terminal, a production platform or a floating loading device. The invention also comprises a device for practicing the method.

[0002] Crude oil normally contains fractions of light and heavier gases which cannot be transported together with the liquid fraction of the crude oil due to their volatility. Transport-technically it would be favourable if the crude oil were separated into a gas fraction, a wet gas fraction and a liquid fraction. However, separation and transport of the petroleum products in three fractions from a production field involve considerable extra expenses, and it is therefore more common for the crude oil to be separated into a gas fraction and a liquid fraction. The wet gas phase is therefore divided into a lighter fraction which is transported together with the gas fraction, and a heavier fraction which is transported with the liquid fraction.

[0003] When a liquid petroleum fraction containing heavier gases, such as propane and butane, is being loaded at a moderate positive pressure, gages continuously evaporate from the liquid fraction. To prevent a pressure increase in the transport or storage tanks, the evaporating gases, the volatile organic compounds, must be drawn from the tanks and be carried away for combustion or they must be carried to a re-injection plant.

[0004] It is known that there is relatively extensive evaporation of volatile organic compounds as oil is being pumped into larger tanks. Normally a pressure in the order of 1.05 to 1.07 bar is maintained in both storage and transport tanks. During the loading of a tanker for example, it is common for the oil to be pumped from a storage tank through a supply pipe to a position above the cargo tank, from where the oil is directed into the tank through a drop line down to the bottom portion of the tank. A drop line of this kind may have a length in the order of several tens of metres.

[0005] When the oil enters the upper end portion of the drop line, gravity will accelerate the liquid flowing down the drop line, whereby, a lower overall pressure is created in the supply pipe and the upper portion of the drop line. In these pipes, in which there is a lower overall pressure, there is considerable evaporation of volatile organic compounds, which will be re-liquefied only to a minor extent when the pressure increases again to the normal pressure of the tank.

[0006] The invention has as its object to remedy the drawbacks of known technique.

[0007] The object is achieved, according to the invention, through the characteristics specified in the description below and in the following claims.

[0008] A preferably non-combustible gas or a mixture of gases, e.g. exhausts from a combustion process containing carbon dioxide, CO₂, and nitrogen, N₂, is pumped in sufficient amounts into the upper portion of the drop line, so as to prevent a negative pressure from occurring when oil is flowing down the drop line into the tank.

[0009] Experiments have shown that the evaporation of volatile organic compounds during loading is reduced, to a considerable extent, by preventing the creation of a negative pressure in the supply pipe and drop line. It is relatively simple to separate, in accordance with known technique the non-combustible gases from the volatile organic compounds in a subsequent separation plan, for gas flowing out of the cargo tank.

[0010] The method according to the invention is equally suitable for use during loading and unloading of ships and other larger tank facilities when oil or oil products such as gas and liquid natural gas are to be moved.

[0011] In the following will be described a non-limiting example of a preferred method which is visualized in the accompanying drawing, in which the figure shows schematically a loading arrangement, in which oil is pumped from a storage tank into a tanker.

[0012] In the drawing the reference numeral 1 identifies a storage tank for oil. A pump 2 is connected to the storage tank 1 through a pipe 4 and to the loading point 8 of a tanker 6 through a pipe 10 and a flexible connection/hose 12.

[0013] A drop line 14 extends from the loading point 8 of the tanker 6 down to the bottom portion of the cargo tank 16 of the tanker 6. A gas outlet 18 extends from the top of the cargo tank 16 to a not shown gas-treating plant.

[0014] A pipe 20 extends from the plant, not shown, for the supply of gas, for example a combustion plant of a design known in itself, through a control value 22 to the loading paint 8 or the upper portion of the drop line 14.

[0015] When oil containing volatile organic compounds is pumped by the pump 2 from the storage teak 1 through the pipes 4, 10 and the hose 12 to the loading point 8 of the tanker 6, non-combustible gas is supplied at the same time through the pipe 20 by way of the valve 22 to the loading point 8 or the upper portion of the drop line 14, where it is mixed with the entering oil. The mixture of oil and non-combustible gas then flows down the drop line 14 into the cargo tank 16 of the tanker 6.

[0016] Through the supply of a suitable amount of preferably non-combustible gas to the loading point 8 or the upper portion of the drop line 14 during the loading of oil, the formation of a negative pressure in the pipes/hose 10, 12 and 14 is essentially prevented. The evaporation of volatile organic compounds during loading is thereby reduced, to a substantial degree. 

1. A method for reducing the evaporation of volatile organic compounds (VOC) during the filling of an essentially liquid petroleum product into a storage and/or transport tank (16) through at least one drop line (14), possibly also through a loading point (8), and where a gas or mixture of gases is supplied to the upper portion of the drop line (14) or the loading point (8) during the filling operation, characterized in that gas or gas mixture that is supplied to the upper portion of the drop line (14) or the loading point (8) during the filling operation is non-combustible.
 2. A device for reducing the evaporation of volatile organic compounds (VOC) during the filling of an essentially liquid petroleum product into a storage and/or transport tank (16) through a drop line (14), possibly also through a loading point (8), and where a supply pipe (20) for a gas or gas mixture is connected to the upper portion of the drop line (14) or the loading point (8) during the filling operation, characterized in that the inlet end of the supply pipe (20) is connected to the waste gas system of a combustion plant. 